Objects can be deselected one at a time by left clicking on the selected objects. A more powerful way to deselect items is to use the Home > Select > Deselect command. This gives you access to most of the selection methods available on the Select panel (Pointer/Window, Intersecting Line, 3D Box, XY Plane, XZ Plane, YZ Plane, Groups, Frame Sections, Cable Properties, Tendon Properties, Area Sections, Solid Properties, Link Properties, Joint Constraints, Labels, All), except that now they are used to deselect rather than to select.
For example, assume that you want to select all of the objects in your model except for those in a particular XZ plane. Do this by first using the Home > Select > All command and then using the Home > Select > Deselect > Coordinate Specification > Click Joint in XZ Plane command.